Methane emanates naturally from coal seams and the surrounding geological strata, and is a significant mine safety hazard. Various studies have been conducted to determine how to dispose of this methane gas and whether the methane can be advantageously utilized.
The methane present within the working areas of a coal mine is currently removed through the drawing of fresh air into the work areas by large ventilation systems. These systems function to dilute and withdraw the methane gas. The gases that emanate from coal beds generally do not contain such pollutants as carbon monoxide, sulfur oxides, and nitrogen oxides, but do contain small quantities, usually below 20% by volume, of heavier alkanes, e.g. ethane, propane, butane, and stable gases, e.g. carbon dioxide, oxygen and nitrogen.
The ventilated coal mine gases, referred to as colliery emission streams, have varying flow rates both with respect to time and with respect to different mines. However, the flow rate of these colliery emission streams generally ranges from between 300,000 and 600,000 standard cubic feet per minute (scfm). The concentration of methane in the stream varies between about 2,500 and 10,000 ppmv, which corresponds to a methane emission rate of about 750 to about 6,000 scfm per mine. In the United States alone, the total emission to the atmosphere of methane from coal mines has been estimated at 10 million scfh, which represents wasted thermal energy of over 9 billion btu/hr. Additionally, the large fans used to sweep air through the coal mine to provide ventilation also consume large quantities of electric power, often between 4,000 and 7,000 kW per mine.
In some cases, ventilation alone proves ineffective in controlling methane concentrations in the deepest and most intricate mines such that other means of removing methane become necessary. One method for dealing with this problem is to drill "drainage holes" into a coal seam prior to the actual mining operation. These drainage holes can yield significant quantities of relatively pure methane, sometimes up to 90 percent by volume while reducing the emission rate of methane in the ventilation air. The rate of drained methane removal is approximately the same as the rate of vented methane emission (1,000 to 5,000 scfm per mine). The drained methane has the potential to be sold as a commercial fuel gas.
The concentration of methane in the colliery emission stream is generally below 2 percent by volume, and more commonly below 1 percent by volume. Thus, conventional combustion systems are unable to take advantage of this relatively large, but dilute, source of energy. For direct combustion, a premixture of methane and air must be in the range of from about 5 to about 15 volume percent methane, the so-called flammable range. This level cannot be reached by combining the methane retrieved from the drainage holes with the colliery vent stream, which combined could lead to a concentration only as high as 2-3 percent by volume. To add sufficient commercial fuel to raise the concentration up to the flammable limit would be prohibitive.
Various other means of utilizing this large, but low concentration by volume of methane gas have been considered, but to date, none has been widely adopted. First, spark ignition engines are potentially able to convert the chemical energy in the methane/air mixture to useful energy, but this technology is not well suited for the task due to the inability of such engines to operate with mixtures below the lean limit thus necessitating large quantities of supplemental fuel. A second alternative is to use a gas turbine engine. However, these engines would also require a substantial quantity of supplemental fuel along with the energy required to compress the supplementary fuel to the operating combustion pressure. A final suggested design alternative is the use of regenerative thermal oxidizers; however, such systems have a relatively high pressure drop and generally operate on a reciprocal flow operation, which presents problems associated with the design of how to connect these units to the ventilation fan ducts.
A need therefore exists to design an alternative system to effectively destroy the methane present in colliery vent streams. A useful design should be able to efficiently convert the energy potential in the colliery vent streams to useful power.